Psy-Kosh, dangerous heuristic. Isn't that how the Nazis thought of the Jews? We should look first and foremost at ways things fit into clusters, not ways they don't - otherwise nine-fingered Fred gets ruled out of being human at an early hurdle. I'm sure you'll agree Fred fits better into 'human' than 'broad general-human-type', despite his missing digit.

Ostriches are a long way from that tight, feathery birdy cluster, but we leave them out of 'general bird-ness' at our peril. Mr Ostrich scores 84% on birdiness, not 16% on not-birdiness. (He also scores in the high 60s in dinosauriness, but that's another matter.)